Dynamic tuning for wave energy conversion

ABSTRACT

Dynamic tuning of the oscillating buoyant body of a wave energy converter over a range of periods that characterize energetic swell is achieved by controlling the ratio of total mass moment of inertia to restoring force and by controlling wave bridging. Mass is varied by locking and unlocking neutrally buoyant masses or by trapping and releasing seawater. For surging, water-plane area is forcibly varied. For rotational variants of heaving and surging and for pitching, the radius of gyration is varied. Invariant mass is reduced by using materials and structures with high ratios of strength to weight. Invariant added mass is reduced by streamlining, by constant radius body profiles and by keeping moving structures out of the water. Added mass is varied by using variable angle fins. Control of wave bridging, for a pitching body, including a wave following device, is by locking and unlocking serial segments of the buoyant body.

RELATED APPLICATIONS

UK patent application GB 1116881.2 filed 30 Sep. 2011, is herebyincorporated by reference.

FIELD OF INVENTION

Oceanic wave energy conversion.

BACKGROUND OF THE INVENTION

Large amounts of wave energy are potentially available, with especiallyfavorable locations in, for example, the UK, Ireland, the USA, Australiaand New Zealand. Wave energy can be produced with low carbon emissionsand can be part of the solution to the risks of global warming.

There has been much interest in wave energy. Around 1,500 wave energypatents have been published worldwide over the last 100 years. Around 50companies worldwide are trying to develop wave power. So far there hasbeen no successful commercial megawatt-scale generation of power fromwaves.

The main obstacles to such implementation are poor reliability, highcost, low efficiency and environmental impact.

Reliability and cost problems arise from the oceanic environment, whichis corrosive, erosive, laden with encrusting life-forms and which, inextreme weather, is capable of delivering massive mechanical stress.

Costs are high where the design of the wave energy converter (WEC)relies on over-engineering to counter extreme conditions. It is betterto have a compliant design that mimics nature's solutions to oceanconditions or to minimize vulnerable elements of the WEC design or touse an adaptive approach where, for example, in extreme weather, the WECsubmerges.

Costs are also high if extensive underwater work is required: forexample to fix a rigid structure to the seabed. If a structure alreadyexists, for example to support a wind turbine, this can be exploited.Structural costs can be reduced by using slack tethering to anchors onthe seabed.

Low efficiency arises especially from non-resonant coupling between theWEC and the wave. In the worst case, the WEC and the wave are out ofphase and energy is subtracted rather than added so that the instantefficiency is negative. Out of phase behavior is guaranteed where thewaves are stochastic. Such waves occur in regions of sea that are (a)sheltered from the major oceans so that waves are mainly driven by localwinds (b) in shallow waters where sea-bed reflection and refractionconfuses the wave pattern (c) close to shore where reflection,diffraction and refraction also confuse the wave pattern. The distortingeffects of seabed and shoreline are increased by irregular geometry.Distortion of wave motion by the seabed is almost zero where the seadepth is greater than half the wavelength. The typical peak of theannual ocean swell energy spectrum is 8-10 seconds period. Thiscorresponds to a half wavelength of around 50-80 meters. Therefore anefficient WEC will be located in ocean that is at least 40 m deep andusually at least 1 km offshore.

An efficient WEC must be located in a region where there are strongpersistent swells: in brief, offshore from a good surfing beach. Usefulswells arise from prolonged central Atlantic and Pacific ocean storms ofthe kind generated by the Coriolis effect midway between the equator andthe poles. Storm waves generated in mid-ocean decompose into persistentswell trains each of uniform period. Since long-period swells travelfaster than short-period swells, the swell trains separate. Such trainsare capable of traveling thousands of miles with minor energy loss. Byusing satellite synthetic aperture radar, swells can be tracked andarrival can be predicted many days ahead.

An efficient WEC will also be oriented to the dominant swell. This canbe done in three ways: (a) by fixing the WEC in a zone where thedominant swell has a persistent direction (b) by using anomnidirectional device (c) making the WEC self-aligning.

Exposure to persistent energetic swells is a necessary but insufficientcondition for an efficient WEC. It is also necessary for the WEC to bedynamically tunable over a wide range, where wide range means the rangeof periods that characterizes energetic ocean swell: around 5 to 15seconds.

Dynamic tuning means tuning during operation and tuning rapidly betweendifferent swell periods. The required speed of tuning depends on theduration of swell trains, the variability of swell period, thepredictability of swell period, the cost of tuning delay and the cost oftuning. The duration of swell trains depends on the duration of thestorms that cause them: for example, around 70% of Atlantic tropicalstorms last more than two days (an average over the last 50 years).

Shifts of swell period tend to be in steps of only one to two seconds.Preferably it should be possible to shift the period of the WEC by onesecond within two minutes. Depending on all the factors mentioned, thepreferable one-second tuning time for a specific configuration of WEC ina specific location may be between 10 seconds and 10 minutes.

Dynamic tuning is useful not only to achieve high efficiency but also toprotect the WEC. When the amplitude of oscillation becomes excessive inextreme seas, the WEC can be quickly detuned.

Low efficiency can also result from single-vector energy capture. Thewater particles of a wave move orbitally, carrying energy in twovectors: vertical and horizontal. A heaving (or bobbing) WEC can captureonly the vertical vector; a surging WEC can capture only the horizontalvector. Such devices have a maximum efficiency of 50%. Two-vector energycapture ie pitching offers a wave-to-WEC energy transfer that is up to100% efficient. Despite the apparent advantage of the pitching vector,heaving and surging WECs represent the majority of WECs offered (in2012) by wave energy companies. This bias may reflect a belief thatheaving and surging WECs offer lower life-cycle costs.

For efficient capture of wave energy the WEC should engage with theswell at or near the ocean surface. At periods that characterizeenergetic swell, the available swell energy falls by around 2% for everymeter of depth. Preferably the WEC should engage the swell within thetop 10 meters.

Lastly, a successful WEC must be environmentally acceptable. This tendsto limit the application of designs that are either on-shore ornear-shore since there are few such locations near to any populationcenters where a large-scale installation would be tolerated.

Difficulty of Wide Range Dynamic Tuning

Although large WEC efficiency improvements are possible using resonantenergy transfer, there has been little progress in achieving this. Theequation for the period of an oscillating floating body helps to explainwhy:T _(B)=2π√((I _(M) +I _(A))/I _(C))  1.Where:T_(B) is the period of the body in seconds.I_(M) is the mass moment of inertia (MOI) in the direction ofoscillation.I_(A) is the added mass MOI of the body in the direction of oscillation.I_(C) is the moment of the coefficient of restoring force.

Note:

-   -   Added mass is the inertia of fluid displaced by a body that is        accelerating or decelerating. For example, a flat-bottomed        vessel in pitching motion displaces a relatively large volume of        water ie the added mass is large. For the same enclosed volume,        a V-shaped body in motion displaces a smaller volume of water ie        the added mass is small.    -   The restoring force is the net buoyancy. The coefficient is the        rate of restoring force. It is proportional to the water-plane        area

For T_(B) to vary by 3×, the bracketed function on the right of equation1 must vary by 9×. At first sight, the solution is obvious: for anygiven value of I_(M) and I_(A), simply vary I_(C). For example, if westeadily submerge a heaving vessel with a conical top, then thewater-plane area will steadily diminish and T_(B) will rise. However,the power from the WEC varies with restoring force, stroke length andperiod. By reducing I_(C), the power capacity is also reduced. Inspecific conditions that are included in the present invention, thisproblem can be overcome. But generally, we want I_(C) to be fixed and wemust vary I_(M)+I_(A).

For I_(M)+I_(A) to vary by 9×, either:

-   -   One of these terms is very small and the other term can vary by        9×, or    -   Both terms can vary by 9×

It is easy to show that if either term is fixed and not small, or ifeither term has a fixed component that is not small, then the requiredvariation in the other term can be much larger than 9×.

If we consider a heaving body, then equation 1 simplifies to:T _(B)=2π√((M+M _(A))/C), where:  2.M is the massM_(A) is the added massC=A×d×g where A is the water-plane area, d is the density of seawaterand g is the gravitational constant.

If, say, the body is a simple upright cylinder, then T_(B) cannot bevaried by 3×, due to the significant fixed value of M_(A). If M_(A) isreduced by streamlining, we may need a 10× variation in M.

An obvious method of varying M is to pump seawater into the cylinder.The amount required for a 10× variation in T_(B) depends on the value ofM when T_(B)=5 seconds.

For example, if the cylinder has a diameter of 5 m, then A=19.6 m² andfrom equation 2, approximately:2π√((M)/19.6 g)=5M=123 tonnes, equivalent to a draft of around 6 m

To raise T_(B) to 15 seconds requires increasing this mass by 10×: onaverage pumping 123 tonnes of water for every second of change in T_(B).This is operationally costly and slow.

The present invention shows how to achieve wide-range dynamic tuningwithout (a) sacrificing power output and (b) at low operational cost andhigh speed.

PRIOR ART

Prior art lacks descriptions of WECs that are dynamically tunable toresonance with ocean swell and tunable over a wide range of swellperiods.

McCabe (U.S. Pat. No. 5,132,550 1992) describes two pontoons on eitherside of a damped, high inertia float, with the pontoons hinged to thefloat. Energy is captured from the relative motion of the pontoons andthe float, using water pumps. The pontoons can be tuned by ‘adjustableballast’ although the exact method is not specified. The pontoons areflat-bottomed, creating a large invariant added mass MOI that makeswide-range tuning impossible.

McCormick (US Pat App 2009/0084296) describes three linked floatsaligned with each other and with the ocean swell. The linkages arehinges and pumps are mounted over the hinges. The length of the systemis selected to be resonant with the average wave period. Tuning aroundthis period uses U-tubes filled with water. The floats are shown asflat-bottomed, creating a large invariant added mass MOI that makeswide-range tuning impossible.

Beane (U.S. Pat. No. 7,755,224 2010) describes a tunable pitching WECthat meets some of the listed requirements. But Beane does not considerthe effect of added mass. He describes a flat-bottomed vessel that istuned by a solid mass moving along a keel.

Laz et al (U.S. Pat. No. 8,198,745 2012) describes a rolling WEC inwhich a tunable pendulum is mounted on a tunable raft. The pendulum isof conventional design and therefore cannot be tuned to long periodswithout an impractically large superstructure. The raft is tuned byextending or retracting flotation tanks that are mounted on its sidesand by pumping seawater in and out of the tanks. The raft and tanks areshown as flat-bottomed. Again the large invariant added mass MOI makeswide range tuning impossible.

In the cases listed above, the resonant period is dominated by a largefixed mass MOI and a large fixed added mass MOI and it is not possibleto tune dynamically to the range of periods that characterize energeticocean swells.

Raftery (U.S. Pat. No. 8,093,736 2012) describes a pitching WEC that isdynamically tuned by adjusting the tension of lines tethering asubmerged platform and of a cable tethering a floating vessel to theplatform. As the vessel rises, the cable is pulled and drives ahydraulic pump. As the vessel falls, stored hydraulic energy is used toreel the cable in. The same system is used for tuning, power take offand the recovery stroke. Applying equation 1 we find that all the termsvary continuously as the cable length changes:

-   -   I_(M): the radius of motion of the vessel changes.    -   I_(A): additionally the drag area changes    -   I_(C): additionally the water-plane area changes.

Variation in cable length is a function of wave height, which isindependent of swell period. The implication is that when wave height isvarying, tuning to the swell period may be difficult or impossible.

Gregory (US Patent Application 2011/0089689) describes a dynamicallytunable pitching WEC capable of tuning over a wide range. The WECachieves robustness by using the relative motion of the body and aninternal compact pendulum, and so avoiding external moving parts. Themass MOI is varied by changing the distribution of solid ballast andseawater ballast. The added mass MOI is varied by varying the draft of abody bottom that tapers from flat at each end to V-shaped in the middle.

A drawback of Gregory's method is that it relies for tuning on variationin draft, which is achieved by pumping large volumes of seawater.

SUMMARY OF THE INVENTION

A WEC is made dynamically tunable to the range of periods thatcharacterize energetic ocean swell by:

-   -   Arranging the fixed mass MOI to be low.    -   Arranging the variable mass MOI to be variable over a wide        range.    -   Arranging the fixed added mass MOI to be low.    -   Arranging the variable added mass MOI to be variable over a wide        range.    -   In the case of a surging body, forcibly varying the water-plane        area    -   In the case of a pitching body, avoiding wave-bridging

Additionally, to maximize efficiency, the WEC aligns with the dominantswell.

Low fixed mass MOI

Referring to equation 1:

I_(M) has a fixed component: for example, the MOI of the fixed mass ofthe floating body and, for tuning purposes also has a variablecomponent: for example the MOI of pumped seawater ballast. To enable thewidest range of tuning, the fixed component of mass MOI should be smalland the variable component should be widely variable.

The fixed component of I_(M) is small if the fixed mass of the body islow. The fixed mass is significant for a rotating body (a pitching bodyor a heaving or surging body constrained to rotate) where a significantportion of fixed mass is remote from the center of rotation. The fixedmass can be of less significance in the case of pure heave or puresurge. The desired tuning range is 5 to 15 seconds: the low end of thisrange can require a body in pure heave or pure surge to have a largemass. Therefore, it can be satisfactory to build a pure heaving or puresurging body in, for example, steel or reinforced concrete.

The fixed mass of a floating body is reduced if:

-   -   The body is made of material having a high specific strength        (ratio of strength to weight). Fiber-reinforced polymer (FRP)        can be used. Glass-epoxy FRP has a specific strength that is 2×        medium carbon structural steel (AISI 1045) and is comparable        with high-tensile aluminum alloy (2045 T4). Such aluminum alloys        can also be used. Kevlar-epoxy FRP has a specific strength that        is around 11× structural steel. The specific strength factor for        carbon-epoxy is 15×. By orienting the fibers in the polymer it        is possible to construct structures with extra strength in a        required direction. Stiffness can be increased by combining        layers with different fiber orientation.    -   A sandwich construction is used. A typical sandwich comprises        two thin stiff strong sheets separated by a rigid honeycomb or a        rigid corrugated sheet or a solid foam. Steel sandwich panels        have been shown to save 50% of weight compared with standard        steel construction. A sandwich construction can be combined with        the use of FRP or aluminum alloy.    -   Stressed-skin structure is used. A high ratio of strength to        weight is a necessity in aircraft and therefore design methods        have evolved to deliver this result. The methods of design of        wings and fuselage are relevant. A typical wing comprises an        upper and lower shell of sandwich construction fixed to an inner        frame comprising spars and ribs. The outer-shell is        load-bearing. The structure of bird bones also provides guidance        to suitable design. Bird bones, which have evolved to maximize        the strength to weight ratio, are hollow, with a thin outer        layer and, in the larger bones, internal struts that merge with        a cellular structure at stress points. In the case of a        wing-like body (see next section: LAM blades) a suitable        structure comprises a tubular spar through which a dense tuning        mass is moved, and ribs fixed to this spar that support a        sandwich shell. In the case of a pod-like body (see next        section: LAM pods), a suitable construction is a sandwich shell        braced by internal struts.    -   Space-frame structure is used: this is a high strength to weight        ratio structure, typically using repeating tetrahedral trusses.    -   The mass of WEC parts fixed inside the floating body is kept as        low as possible. For example, the power capture and conversion        and storage system is kept, where possible, elsewhere: for        example on an adjacent non-moving body.

Where I_(M) is rotational, the need for reduction in fixed mass varieswith the square of the distance from the axis of rotation. A hybridconstruction is possible, in which the near-axis portion of the floatingbody is, for example, a conventional steel construction and the tips ofthe body are made of advanced composite.

Where a floating body is tuned by moving a mass along a rotating arm, itis also an advantage for the arm to have a high strength to weightratio. Therefore the same methods listed above also apply to the arm.

A structure that uses some or all of these methods to reduce the fixedmass MOI is called here a LM (low mass) structure.

Variable Mass MOI

The variable component of I_(M) in the case of simple linear oscillation(for example, a heaving WEC) appears at first sight to be limited toseawater ballast that is pumped in and out of the floating body.However, I_(M) can be varied without pumping seawater by locking andunlocking external masses to the body. In an example, a rigid verticalspar is fixed to the bottom of the floating body. Below the body andsliding on the spar is a neutrally buoyant annular mass. A mechanisminside the annular mass provides a variable friction grip between rodand spar. By activating this mechanism, the annular mass becomes coupledto the body. More then one annular mass can be placed one above theother on the spar. Stacked masses allow the tuning range to be extended.In a variant of this method, annular masses are a stack of sleevesaround the floating body.

This method can be generalized to any variably compliant couplingbetween an oscillating floating body and neutrally buoyant masses. Forexample, a neutrally buoyant annular mass encircles a heaving floatingbody and is coupled to the body by variably compliant attachments:

-   -   Cables. When these are loose, the body moves freely inside the        annulus. When these are tight, the body and the annulus are        fully coupled.    -   Hinged arms. The motion of the arms is constrained by hydraulic        pistons and, by turning a valve in a hydraulic circuit, the        resistance of the pistons is varied. When the valve is fully        off, the arms are locked in place and the body and the annulus        are fully coupled.

Where the floating body is rotating, the mass MOI can be varied bychanging the radius of a movable mass about the axis of rotation. Byusing an LM structure, by using a dense movable mass and by using a longarm it is possible to achieve a variation in mass MOI of 9×. In effectthe radius of gyration is changed by 3×. The method of moving the massshould avoid fixed mass MOI. For example, the mass can be moved along atoothed track by an electric engine mounted on the mass. In anotherexample, the mass is moved along a rail by a cable and the drivingengine is located close to the axis of rotation.

An obvious application is to a tune a pitching floating body. But it isalso possible to convert a surging or heaving body to rotation and souse a varying radius of gyration to tune the floating body. For example:

-   -   A heaving body is attached to a LM arm. The arm rotates on an        axis fixed on a fixed or pseudo-stationary platform. A dense        mass is moved along the arm to and from the axis. The arm can        extend beyond the heaving body.    -   A surging body is fixed to a LM arm. The arm rotates on a fixed        axis. A dense mass is moved along the arm to and from the axis.        The arm can extend beyond the surging body.        Low Fixed Added Mass MOI

Referring to equation 1:

I_(A) has a fixed component and, for tuning purposes also has a variablecomponent. To enable the widest range of tuning, the fixed component ofadded mass MOI should be small and the variable component should bewidely variable.

The fixed component of I_(A) is small if the fixed added mass of thefloating body is low.

The fixed added mass of a pitching body can be negligible if the bodyhas a fixed radius or radii about the pitching axis. Possible geometriesinclude a sphere, a cylinder, a disc, a pair of cones joined at the baseor compound shapes comprising constant radius elements. Only thesubmerged section of the body requires this geometry.

Fixed added mass is small if the body is streamlined. Low added mass fora given body volume is achieved by an ellipsoid cross-section in thedirection of motion through the water and the fineness ratio, FR (ratioof maximum width to length) is between 3 and 7 and ideally 4.5. Thereduction in added mass compared with a blunt profile is as much as 94%.A variation on this shape is a lenticular (double convex lens shape)cross-section. Such an ellipsoid or lenticular cross-section is calledhere a low added mass (LAM) profile. A beam-shaped body with a LAMprofile aligned to the direction of motion is called here a LAM blade. Arotated LAM profile can be described as spindle-shaped or fusiform. Afusiform body aligned to the direction of motion is called here a LAMpod.

Low added mass requires less energy to accelerate a body through water.It follows that low added mass geometry has evolved in aquatic animals,especially oceanic animals that rely on speed and acceleration forpredation.

Most fish have FRs between 3 and 7. Swordfish (average FR=4.2) are amongthe fastest fish, with underwater burst speeds up to 60 kph. Great whitesharks (average FR=5) have burst speeds up to 40 kph. Pinnipeds (membersof the seal family) have FR in the range 3 to 6. The fastest pinniped,the Steller sea lion (average FR=5.5). can reach 40 kph. Cetaceans(whales and dolphins) have FRs in the range 3 to 11. The fastestcetacean is the killer whale (average FR=4.3) with a top sprint speedunderwater of 55 kph.

Low added mass in a pitching body can be achieved by combiningconstant-radii geometries with LAM blades. For example, a pitchingcylinder can have LAM blades fore and aft. Low added mass in a heavingor surging body can be achieved with LAM blades or LAM pods.

Another way to achieve low added mass is to keep moving structures outof the water. For example, a pitching cylinder can have LM arms mountedabove it. Dense masses can be moved on these arms and neither masses norarms incur significant added mass. (Some aquatic animals use the samestrategy: leaping or indeed flying out of the sea).

Variable Added Mass MOI

The variable component of I_(A) in the case of linear oscillation is theadded mass created by variation in the external geometry of the floatingbody. 9× variation in added mass is possible using external fins thatare symmetrically mounted on rotatable shafts fixed in the body wall.When these fins are aligned with the direction of motion the added masseffect is small. The fins can be rotated to present a flat faceperpendicular to the direction of motion, increasing the added mass andtherefore increasing the period of oscillation of the floating body. Toincrease the effect, the fins can be circular and rotate within themiddle of cylindrical enclosures. When the fins are aligned with thedirection of motion, the enclosures offer little resistance to flow.When the fins are locked at right angles to the direction of motion, theenclosure acts as an entraining lip, so increasing the added mass. Byusing long enclosures, water is trapped when the fin is at right anglesto the direction of motion. The effect is to increase not only the addedmass but also the mass. We can call this inertial trapping. Usingsufficiently large enclosures and fins it is possible to quickly varythe total mass by 9×. The apparent price for this is a large amount ofenergy lost to drag.

But ‘inertial trapping’ can be achieved without significant drag:

-   -   A rigid vertical spar is fixed below a heaving body.    -   Mounted on this spar is an open vertical tube.    -   Above this tube and aligned with it is a LAM pod also mounted on        the spar and with an external diameter slightly smaller than the        inside diameter of the tube.    -   The separation between pod and tube allows water to flow freely        around the pod and through the tube. There is little drag.    -   By bringing the pod and tube together, the end of the tube is        blocked, so trapping the inertia of the water contained in the        tube. But the pod provides streamlining and so there is still        little drag.    -   A LAM pod below the tube is used in the same way.

We can call this LM inertial trapping.

Variable angle fins can be mounted on heaving, surging or pitchingbodies. Such fins can have a strong effect on rotating bodies: byplacing the fins at an extreme radius from the axis of rotation, theadded mass MOI can be widely varied. The trade-off is that the finassembly represents a significant fixed mass MOI. The fin assemblyshould be as light as possible. For example, the fins can be actuated bycompressed air, using a compression source that is close to the axis ofrotation.

Other methods of varying the added mass include:

-   -   Retracting fins mounted in sockets in the exterior wall of the        floating body. The fins slide in the sockets, being driven by,        for example, double acting compressed air pistons. When        retracted, the fin ends are level with the exterior surface of        the body. When extended, the fins increase the added mass. The        extension can be variable. Other geometries can be used.    -   Adding external added mass to the body. For example, a heaving        floating body can have a variably compliant attachment to a        neutrally buoyant annular mass. The annular mass has a lipped        horizontal skirt that creates significant added mass. When the        attachment is loose, the annular mass has no influence on I_(A).        When the attachment tightens, I_(A) rises (and T_(B) increases).        We can call this variable compliance tuning.

In other examples of variable compliance tuning:

-   -   A heaving body has hinged fins fixed symmetrically to its        circumference. The hinges are fixed to the body and allow the        fins to rotate through (substantially) 180 degrees. At 90        degrees, the fins are horizontal ie perpendicular to the heaving        motion. When fully compliant, the fins rest flat against the        body as the body moves. When the body reverses direction, the        fins rotate and flatten against the body in the opposite        direction. The added mass effect is small. By inflating air bags        on each side of each fin, the fins are prevented from flattening        against the body at each reversal of motion and so the added        mass rises. When the bags are fully inflated, the fins are held        at 90 degrees and the increase in added mass is at a maximum.    -   A pitching LAM blade has a pair of flexible fins fixed        symmetrically to it at the fore and at the aft. The fins have a        horizontal surface. For example, the fins are made of molded        neoprene reinforced with steel and/or aramid fibre cord. The        fins contain air chambers that can be inflated or deflated. When        the chambers are deflated, the fins are compliant and bend with        the movement of the LAM pod so that the increase in added mass        is small. When the chambers are inflated, the fins are stiff and        the increase in added mass is large.        Forcibly Varying the Water-Plane Area

As already discussed, it is easy but counter-productive to tune afloating body by reducing the water-plane area. However, it is notcounter-productive to increase the water-plane area. This can be done bypulling the body downwards, for example, using a cable and winch. Sincethis inhibits vertical motion, this method is limited to surging bodies.Given a sufficiently buoyant body and appropriate geometry, it ispossible to increase the water-plane area by 9× and so tune the bodyfrom 15 to 5 seconds period.

As examples:

-   -   A surging floating body is slidably mounted on arms fixed to the        seabed. The arms move in the direction of the surge by sliding        or rotating. Power is extracted from this motion. The body is        tuned by pulling the body down the arms, using, for example, a        cable driven by a highly geared locking winch.    -   A surging floating body is anchored by a tensioned cable to the        seabed. The tension-on the cable is varied by a locking winch        that is preferably inside the body. The body is omnidirectional        ie it has radial symmetry and has a radially symmetrical PTO        system. In an example, the PTO system comprises three        oscillating shaft generators (OSGs) driven by cables anchored at        one end to the seabed and at the other end carrying a        counterweight. OSGs are explained later under PTO systems.        Avoiding Wave-Bridging

Wave-bridging limits the dimensions of all WECs. For example, theomnidirectional surging body just described should not have a diametermuch greater than the wavelength corresponding to the smallest designvalue of T_(B). For T_(B)=5 seconds, the limiting diameter is around 40m. Above 40 m, the body starts to bridge the peaks of successive swellsand at 60 m, motion at T_(B)=5 seconds is suppressed.

The problem of wave-bridging is acute for pitching bodies. Rationally, apitching body is optimized for the peak annual swell energy: usually ForT_(B)=8 to 10 seconds.

The optimum length is between half and one wavelength: for T_(B)=10seconds this is between 80 m and 160 m. But at 80 m, wave-bridgingoccurs for values of T_(B) below 6 seconds.

Solutions to this problem are:

-   -   Limiting the length of the pitching body to around 40 m: this        will not bridge any swells from 5 to 15 seconds period. The body        is tuned over this range of periods by a combination of        -   LM structure to minimize fixed mass MOI        -   LAM profile to minimize fixed added mass MOI        -   Out of water structure to minimize fixed added mass MOI        -   Moving dense masses to vary the mass MOI        -   Variable angle fins to vary the added mass MOI    -   Using gravitational restoring force instead of buoyant restoring        force. This avoids the need to immerse buoyant arms and so        avoids wave-bridging for any value of T_(B). The floating body        has fixed below it a LM and LAM keel with a mass at the end. The        keel mass provides gravitational restoring force. Fixed above        the body and out of the water are symmetrical LM arms. The body        is tuned by moving dense masses symmetrically on these arms.    -   Locking and unlocking the links between a series of body        segments to modify the effective rigid immersed length of the        pitching body. The locking method can be conveniently combined        with use of an OSG power take off. The link between segments        employs a horizontal shaft that passes through the fore section        of an aft segment. The shaft turns on bearings in the wall of        the aft segment. Outside the aft segment, the shaft is rigidly        connected to a rigid yoke. The yoke rotates in the horizontal        plane on a hinge fixed to the aft section of the fore segment.        In effect the two segments are joined by a universal joint. An        OSG in the aft segment converts the oscillating rotation of the        shaft into useful electrical power. The shaft is locked by means        of a disc brake: the disc turns on the shaft and is bracketed by        brake pads powered, for example, by compressed air. This method        is discussed further in the next section.

Other locking methods are possible. For example:

-   -   The body segments have opposing faces that are shaped to fit        each other. For example, the aft face of the first segment is        concave in the shape of a cone and the fore face of the second        segment is convex in the shape of a cone. A cable runs from the        axis of the concave face to the axis of the convex face. By        tightening the cable, the faces are pulled together and the two        segments lock.    -   The aft face of the first segment is flat and the fore face of        the second segment is also flat. Mounted on both faces are        multiple alternating plates of steel and rubber. A cable runs        from the axis of the aft face to the axis of the second face. By        tightening the cable, the segments are pulled together, the        plates are pressed together and the segments lock.        Wave-Following

A wave-following WEC uses a series of linked floating bodies andextracts power from the relative motion of adjacent bodies. The bodiesmove in both the heave and surge vectors and therefore are pitching,although the axis of rotation can vary. At maximum power, adjacentbodies are each half a wavelength long and the link between bodiesoscillates to maximum amplitude, with the center of each body tending tostay level. Therefore a tunable wave-following WEC can be designed as aseries of symmetrical pitching bodies, with a LM and LAM bladeconstruction, dense masses that can be moved symmetrically from thepitching axis and variable angle fins at the extremes of the blade. ButLM design is contradicted by the significant fixed mass of the linksbetween bodies and the associated PTO systems. One solution is to movethe main mass of the PTO system to the pitching axis. This can be doneby using an Oscillating Shaft Generator (OSG) (see next section): theshaft is turned by the link with the adjacent body and a cable runs fromthe shaft to the OSG power unit at the pitching axis.

The lengths of the bodies of a wave-follower are critical. Ideally allthe bodies are the length of half a wave-length at values of T_(B) from5 to 15 seconds. In principle, this can be done by using a series ofnine bodies that are each 20 m long. The links between the bodies arelockable. When all links are unlocked, all the bodies are at the optimumlength for T_(B)=5 seconds. When all the links are locked, the combinedlength is optimum for T_(B)=15 seconds. Intermediate locking andunlocking provides tuning by length for intermediate values of T_(B).

But this is an expensive solution since every link requires a PTO. Amore practical solution is to use only two or three links while ensuringthat there is a linking combination that gives the optimum length forthe peak annual energy of the location of the WEC. For example, if thevalue of T_(B) that corresponds to the peak energy is 9 seconds then itis desirable to have an option to create a rigid immersed length ofaround 65 m.

Alignment with the Dominant Swell

Tuning to the dominant swell is not useful unless the floating body isaligned to the swell. Applicable methods of alignment are:

-   -   Giving the floating body a fixed orientation where the dominant        swell direction varies little: this is sometimes the case for        in-shore WECs. The present invention describes two in-shore WECs        with fixed orientation    -   Designing the floating body to be omnidirectional by giving it        radial symmetry. This is almost automatically true for heaving        bodies. The present invention describes a heaving body and also        an omnidirectional surging body. In some designs of surging        bodies, the only way to achieve radial symmetry is to use        multiple surging bodies: for example, arranged (in plan view) as        a cross or a square.    -   Making the body self-aligning. This is unnecessary for heaving        bodies, highly problematic for surging bodies and necessary for        pitching bodies, including wave-followers. The obvious way to        self-align a pitching body is to tether the body from the fore        section. This can be combined with use of an OSG as follows:        -   As already described under wave-following, the OSG (see next            section) power unit is placed near the pitching center.        -   The OSG is driven by a cable that runs to an oscillating            horizontal shaft that passes through the interior of the            fore section of the body.        -   Outside the body, both ends of the shaft are fixed to a            rigid yoke that passes under the body.        -   The rigid yoke enables orbital motion of the body by means            of connection to a crank arm.        -   The crank arm comprises two rigid and buoyant sections. The            crank sections are joined by a vertically acting hinge. The            lower crank section is fixed to a submerged            pseudo-stationary platform by a combination of universal            joint and horizontal rotary bearing. This configuration            provides compliance to roll and yaw and enables the crank            arm to swing round as the swell direction changes.        -   Where the yoke is directly connected to a shaft at the            pitching centre, a rigid LM vane is attached to the aft of            the pitching body.            Power Take Off (PTO) System

The tuning methods described in the present invention concern tuning afloating body to the dominant swell to improve the efficiency oftransfer of energy from energetic swell to floating body. There is alsoan opportunity to tune the power take off to the floating body. This isa separate topic, not covered here.

For present purposes, the PTO system can be chosen from a wide range ofoptions, including systems that are:

-   -   Mechanical: crank, rack and pinion, ratchet wheel and so on.    -   Hydraulic: involving different fluids, pressures, pumps and        motors.    -   Pneumatic: many kinds of pump.    -   Electromagnetic: for example, a linear generator.    -   Piezo electric.

It can be argued that a mechanical system is preferable because it doesnot involve the contamination and reliability risks of hydraulics or thecompression losses of hydraulic and pneumatic systems. It is not anaccident that most wind turbines use mechanical drives and that so farwind turbines based on hydraulics have not been a success.

The PTO examples used here are all oscillating shaft generators (OSGs).The OSG takes useful power from an oscillating shaft in the followingsteps:

-   -   A gear wheel rotating on the shaft transfers rotation to a        ratchet wheel to achieve one-way motion.    -   The ratchet wheel rotation is transmitted through a gear chain        that greatly increases the velocity of rotation.    -   High-speed rotation is delivered to a flywheel via an        overrunning clutch.    -   The flywheel delivers smoothed high-speed rotation via a clutch        to a rotary electrical generator

By arranging a pair of ratchet wheels and a reversing gear, it ispossible for both directions of rotation to deliver power to theflywheel. It may be convenient to place the flywheel on the oscillatingshaft, rotating freely there.

The OSG is in a water-tight housing and the oscillating shaft is drivenby an arm or wheel outside the housing. For example, the oscillatingshaft passes through a bearing in the wall of the floating body.Examples are:

-   -   An OSG is mounted in a floating body and driven by cable and        counterweight. A cable is looped over an external wheel on the        shaft of the OSG. One end of the cable is attached to the        seabed. The other end is attached to a counterweight. As the        floating body oscillates relative to the seabed, the cable        spools over the wheel and the shaft oscillates. The same method        can be used to capture the relative motion of any two bodies.        The OSG can be placed on an external platform and the end of the        cable can be attached to the floating body.    -   An OSG is mounted in a floating body. The OSG shaft is connected        rigidly to an arm and the arm is hinged to another stationary or        out of phase body. The shaft is again driven by relative motion.        In a variant, as already outlined, the oscillating shaft is        remote from the rest of the OSG system and connected to it by,        for example, a cable. By this means, the main mass of the OSG        can be kept near the pitching axis of a pitching body, and, at        the same time, the body can be tethered by an oscillating shaft        in the fore section of the body.        Control System

Dynamic tuning of a WEC requires a control system. The relevant inputsto the control system include:

-   -   Immediate readings of incident swell height, direction and        period using, for example, on-board pressure gauges and        accelerometers.    -   Reports of the same data from local monitoring buoys.    -   Reports of the same data from satellites.

If the expected or actual conditions appear imminently dangerous, thecontrol system takes two steps:

-   -   Detuning the vessel to reduce response.    -   Flooding ballast tanks to submerge or semi-submerge the WEC.

The information on swell period is used to adjust the tuning of thevessel to achieve resonance. The control software acts usingpredetermined tables of appropriate tuning settings but records theresults achieved and continuously modifies the tables.

BRIEF DESCRIPTION OF DRAWINGS

Unless stated otherwise, all figures are schematic and not to scale. Insummary the attached figures are:

FIG. 1: a side view of a heaving body with oscillating shaft generator(OSG)

FIGS. 2a to 2c : a side view of a heaving body that is tuned by lockingand unlocking masses.

FIGS. 3a to 3d : examples of LAM profiles shown to scale.

FIG. 4a : a side view in cross-section of the mass locking mechanism.

FIG. 4b : a plan view in cross-section of half of the mechanism.

FIG. 5a : a side view of a heaving body with variable angle fins in thelow added mass position.

FIG. 5b : a side view of a heaving body with variable angle fins in thehigh added mass position.

FIGS. 6a to 6f : a side view in cross-section of variable angle fins indifferent positions and in different enclosures.

FIGS. 7a and 7b : side view of LAM inertial trap before and aftertrapping.

FIG. 8a : front view of a sliding surging blade with tension tuning.

FIG. 8b : side view of the same with cut-away showing cable wheels.

FIG. 8c : plan view of the blade and arms showing sliding arrangement.

FIG. 9a : side view of sliding surging WEC with cutaway showing tensioncable connected to an on-shore winch.

FIG. 9b : side view of sliding surging WEC with cutaway showingpower-transmitting cable connected to an on-shore OSG.

FIG. 10a : side view of a rotating LAM blade surging WEC.

FIG. 10b : plan view showing blade cross-section.

FIGS. 10c and 10d : side views showing tuning with movable internalmasses.

FIG. 11a : side view in cross-section of omnidirectional surging WEC.

FIG. 11b : plan view in cross-section showing arrangement of multipleOSGs.

FIGS. 12a and 12b : side views in cross-section of pitching LAM bladestuned by movable masses.

FIG. 12c : front view in cross-section of LAM blade.

FIGS. 12d and 12e : plan views of pitching LAM blade tuned by variableangle fins.

FIG. 12f : side view of pitching LAM blade showing location of fins.

FIG. 13a : plan view of pitching WEC comprising a cylinder with LAMblades attached.

FIG. 13b : side view of the same showing tethering by crank arm topseudo-stationary (PS) platform.

FIG. 13c : plan view of PS platform.

FIG. 14: side view of pitching cylinder WEC showing out-of-watersupport.

FIG. 15a : side view of pitching cylinder WEC with gravity restoringforce.

FIG. 15b : plan view of pitching cylinder WEC with gravity restoringforce.

FIGS. 16a to 16d : side views of different configurations of awave-following WEC.

FIG. 16e : plan view in cross-section of a segment locking mechanism.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1: side view of a heaving body with oscillating shaft generator

A LAM heaving body (10) floats in a swell (11). The bottom of the body(10) at the vertical axis is fixed to a rigid vertical rod (12).Connected to the rod (12) are at least three slack mooring lines (twoare shown: 13) arranged at equal radial angles (not shown). The lines(13) are fixed to the seabed (14). Fixed to the bottom of the rod (12)is a LAM pod (15) containing an OSG (not shown). The OSG is driven by acable (16) that loops round a wheel (not shown) on the OSG shaft (notshown). The cable is fixed to the seabed (14) at one end and to acounterweight (17) at the other end.

The mooring arrangement permits the body (10) to heave. As it does sothe cable (16) reciprocates and the OSG generates electrical power.

FIGS. 2a to 2c : side views of a heaving body that is tuned by lockingand unlocking masses.

FIG. 2a : a LAM heaving body (10) rides substantially above thewaterline (20). The bottom of the body (10) at the vertical axis isfixed to a rigid vertical rod (12). Not shown, but fixed to the bottomof the rod (12) (see FIG. 1) is a LAM pod containing an OSG. Not shown,but attached to the rod (12) are slack mooring lines permitting verticalmotion of the body (10). Below the body (10) and sliding on the rod (12)are a first lockable mass (21) and a second lockable mass (22). Thesemasses (21 and 22) are shaped as LAM pods. The rod (12) passes throughthe vertical axis of the lockable masses (21 and 22) and in thisinstance, the lockable masses (21 and 22) can move freely on the rod(12).

The lockable masses (21 and 22) have neutral buoyancy and by sensing theposition on the rod by means not shown, a trimming tank (not shown) ineach lockable mass (21 and 22) is used to hold the masses (21 and 22) ata desired depth.

The period of oscillation of the heaving body (10) is a function oftotal mass plus added mass of the heaving body (10) and its connectedparts divided by the water-plane area. Since, by design, the added massis small and the water-plane area is fixed, the device can be tuned byvarying the total mass. In this instance, by design, the period ofoscillation is 5 seconds.

FIG. 2b : This is the same WEC as described under FIG. 2a . The firstlockable mass (21) is locked on to the rod (12). The increase in totalmass is indicated by the greater immersion of the body (10).

In this instance, the first lockable mass has a mass that is 3× thetotal mass of FIG. 2a . As a result, the total mass is increased 4× andthe period of oscillation is increased by 2× to 10 seconds.

FIG. 2c : This is the same WEC as described under FIG. 2a . Both thefirst lockable mass (21) and the second lockable mass (22) are locked onto the rod (12). The increase in total mass is indicated by the greaterimmersion of the body (10).

In this instance, the second lockable mass has a mass that is 5× thetotal mass of FIG. 2a . As a result, the total mass is increased 9× andthe period of oscillation is increased 3× to 15 seconds.

In this example, two lockable masses are used and a 10 second period ofoscillation is chosen as a midpoint. Efficiency of energy capture isimproved if the midpoint is at the peak of the annual wave energyspectrum: this will depend on location. Efficiency is also improved ifsmaller increments of tuning are allowed. For example, if three lockablemasses are used with masses that are 2×, 3×, and 4× the starting totalmass, then the possible steps in total mass are:

1+2=3

1+3=4

1+4=5

1+2+3=6

1+2+4=7

1+3+4=8

1+2+3+4=10

Finer tuning is possible between increments by varying the added mass.Added mass is not a preferred means of achieving major tuning incrementsbecause the price of increased added mass is increased losses of energyto drag. Tuning by means of variable added mass is shown in FIGS. 5 and6.

The lockable masses (21, 22) are preferably the least expensive massesthat can be easily formed into the shape of a LAM pod. At the same time,there should be easy access to the interior. A possible solution is tocast the pod in reinforced concrete in two halves (split along the longaxis); these halves are then bolted together.

Maintenance can be carried out by attaching the halves to floats,unbolting the halves and removing them.

FIGS. 3a to 3d : examples of LAM profiles shown to scale.

FIG. 3a shows the cross-section of a lenticular LAM blade body with afineness ratio (FR) of 3. This body has low added mass in the directionof the long axis. Added mass increases rapidly as the FR is reducedbelow 3.

FIG. 3b shows the cross-section of a lenticular LAM blade body with afineness ratio (FR) of 4.5. This body has the lowest added mass for agiven contained volume.

FIG. 3c shows the cross-section of a lenticular LAM blade body with afineness ratio (FR) of 7. This body has low added mass. Added massincreases significantly as the FR is raised above 7.

FIG. 3d shows the cross-section of an ellipsoid LAM blade body with afineness ratio (FR) of 4.5. This is an alternative to the lenticularshape.

The cross-section of a LAM pod body is the same as shown in FIGS. 3a to3d : if a LAM cross-section is spun on its long axis, then the shapethat is traced out is a LAM pod. If a LAM cross-section is spun on itsshort axis, then the shape that is traced out is a LAM disc.

FIG. 4a : side view in cross-section of the mass locking mechanism.

The rod (12) passes through the long axis of the lockable mass (21, 22),which has a LAM pod profile. Fixed inside the mass (21,22) at each endand abutting the rod (12) is an annular coil (40) that detects therelative motion of magnets (not shown) embedded in the rod (12). Fixedinside the mass (21,22) at the top are a trimming system (41) and acontrol system (42). The trimming system (41) comprises elements thatare not shown and include: seawater ballast tank, electric aircompressor, air reservoir, connecting piping and valves. Air is suppliedthrough a flexible tube from a snorkel floating on the sea surface.Electricity is supplied via a flexible cable from batteries inside theheaving body (10). The control system (42) (detail not shown) acts oninput current from the annular coils (40) to either admit seawater intothe ballast tank or to blow water out with compressed air. The controlsystem (42) also accepts commands from a system (not shown) in theheaving body that monitors swell period and height and also receivesremote commands. As the swell period changes, the control system (42)can cause locking and unlocking of the mass (21,22) to the rod (12).

Wrapped round the rod (12) are two rigid brake pads in the form of asplit cylinder (43). The pads (43) are fixed to rigid pins (44) thatslide inside slots (45) (shown in FIG. 4b ).

FIG. 4b : plan view in cross-section of half of the locking mechanism.

A single brake pad (43) is shown retracted from the rod (12). Asupporting pin (44) is shown inside a slot (45). The pin is connected toa double-acting piston (46) that slides on seals (not shown) inside achamber (47). Compressed air lines (48) are connected to the chamber(47) on each side of the piston (46). The control system (42) operatesvalves (not shown) in the air lines (48) to admit high-pressure air orto release air, so operating the brake.

Compressed air brake technology is well understood and is currentlyused, for example, in commercial road vehicles of up to 7000 kg weight.

FIG. 5a : side view of a heaving body with variable angle fins in thelow added mass position.

A heaving body (10) floats in a swell (11). Fixed below the body (10) isa LAM pod (50) containing actuators (not shown) that control the angleof four fins (51) with a LAM profile. In this instance the fins (51) arevertical so that the added mass effect on the heaving system is verysmall.

FIG. 5b : side view of a heaving body with variable angle fins in thehigh added mass position.

This is the same system as shown in FIG. 5a . In this instance the fins(51) are horizontal so that the added mass effect on the heaving systemis large. By varying the angle of the fins (51), added mass can becontinuously and quickly varied.

It is necessary to have (at least) four fins (51) equally spaced aboutthe vertical axis of the actuator pod (50). By simultaneously rotatingadjacent fins (51) in opposite directions, a net twisting force isavoided.

The actuators can be powered by electricity or by compressed air, bothof which are available for controlling the lockable masses (21, 22).

FIGS. 6a to 6f : a side view in cross-section of variable angle fins indifferent positions and in different enclosures.

FIG. 6a shows a simple LAM fin (51) rotating on a shaft (60). The arrows(61) indicate heaving motion as a result of the swell (11). The fin (51)is in the low added mass position.

FIG. 6b shows a simple LAM fin (51) rotating on a shaft (60). The fin(51) is in the high added mass position.

FIG. 6c shows a LAM fin (51) rotating inside a short enclosure with thinstraight walls (62). If the enclosure (62) is tubular then the fin (51)must be circular. The overall added mass is low.

FIG. 6d shows a LAM fin (51) inside the short enclosure (62) in the highadded mass position. The effect of the enclosure (62) is to increase theadded mass.

FIG. 6e shows a LAM fin (51) rotating inside a long enclosure (63) withthin straight walls. The overall added mass is low.

FIG. 6f shows a LAM fin (51) inside the long enclosure (63) in the highadded mass position. The added mass is increased and also there issignificant trapping of mass.

It can be seen that by placing the fin in an enclosure, the inertialeffect on the period of oscillation of a body can be increased.

FIG. 7a : side cross-section view of LAM inertial trap before trapping.

A vertical rod (70) is fixed to the base of a heaving body (not shown).Fixed on the rod (70) using struts (71) and forming an annulus around itis a thin-walled LM open tube (72). A first LM and LAM pod (73) rides onthe rod (70) above the tube (72) and at a distance that does not impedethe free flow of water through the tube (72). Likewise a second LAM pod(74) rides on the rod (70) below the tube. The arrangement causes verylittle drag.

FIG. 7b : side cross-section view of LAM inertial trap after trapping.

The pods (73, 74) are moved to the tube (72) and close it at both ends,so trapping the inertial mass of the contained water. The ends of thepods (73, 74) provide streamlining and so drag remains low.

As an example, the pods (73,74) are neutrally buoyant and each is movedby a small electric motor (not shown) that drives friction wheels (notshown) on the rod (70).

Use of LAM inertial traps is an alternative to the use oflocking/unlocking masses to control total mass.

FIG. 8a : front view of a sliding surging blade with tension tuning.

A rigid LAM blade (80) is held between two rigid vertical arms (81) andat the top of the arms (81). The blade (80) and the arms (81) arebuoyant. The arms (81) project above the waterline, indicated by theswell (11). The blade (80) is partly immersed. The arms (81) arearranged so that the blade (80) is at right angles to the direction ofswell. (This is an inshore device located so that the direction of theswell varies only slightly). The arms (81) are fixed at the base to anupper shaft (82) held in bearings (not shown) that are fixed in thefoundations (83). Fixed to the upper shaft (82) and rotating with it isa central power wheel (84) around which a power-transmitting cable (85)is wrapped. Also held in bearings (not shown) fixed in the foundations(83) is a lower shaft (86). Fixed to the lower shaft (86) and rotatingwith it are two position wheels (87): one under each arm (81). Wrappedround each position wheel (87) are position cables (88). These cables(88) run up slots (not shown) in the arms (81) and are fixed to the endsof the blade (80). The foundations (83) are fixed to the seabed (14).

FIG. 8b : side view of the same with cut-away showing cable wheels.

The arrows (89) indicate surge direction. The blade (80) is tilted withrespect to the arms (81). The raised face of the blade (80) faces theincoming surge. The cutaway shows the power wheel (84) and thepower-transmitting cable (85). Also shown are a position wheel (87) anda position cable (88).

FIG. 8c : plan view of the blade and arms showing sliding arrangement.

The blade (80) has rails (810) that slide up and down in slots (811) inthe arms (81). The arms (81) are streamlined in the direction of surge.The position of the blade (80) in the slots (811) is determined by theposition cables (88), that run up the slots (811) and are fixed to theends of the blade (80) (not visible in this view).

FIG. 9a : side view of sliding surging WEC with cutaway showing positioncable connected to an on-shore winch.

A position cable (88) is shown running from an arm (81), round aposition wheel (87) and up to a winch (90) mounted on a housing (91)fixed to the shore (92).

There are two position cables (88). These are joined by a yoke (notshown) so that a single cable is operated by the winch (90).

FIG. 9b : side view of sliding surging WEC with cutaway showingpower-transmitting cable connected to an on-shore OSG.

A power-transmitting cable (85) is shown wrapped round the power wheel(84) and also wrapped round a wheel (93) that turns on a shaft (94)mounted in a housing (91) on the shore (92). The shaft (94) drives anOSG (not shown).

When the blade (80) is struck by incoming surge, the blade (80) and thearms (81) are rotated on the upper shaft (82). The assembly continues torotate until the surge weakens and the buoyancy of the blade (80) andarms (81) becomes the dominant force. As the surge reverses, theassembly reverts to the upright position. The reversing surge thenpushes the blade (80) in the opposite direction. The cycle repeats. Theoscillation of the upper shaft (82) is transmitted by the cable (85) tothe shore-based OSG.

By operating the winch (90), the position cables (88) are used to adjustthe height of the blade (80) in the arms (81). This allows the bladeheight to be adjusted for differing tidal levels. The cables (88) alsoenable tuning over a wide range. By design the mass MOI of the blade(80) and the arms is large. The mass MOI is dominant because a highadded mass MOI implies drag losses. At the same time, the blade (80) andthe top of the arms (81) are highly buoyant. As a result the period ofthe device is 15 seconds, but the water-plane area of the blade (80) andthe arms (81) is small relative to the maximum possible water-planearea.

By pulling the blade (80) down, the water-plane area is increased by 9×.This reduces the period to 5 seconds.

Since the distance of adjustment of the blade (80) is small, the winch(90) does not need to be high-powered. A small locking winch willsuffice, operating through a large gear train.

FIG. 10a : side view of a rotating LAM blade surging WEC.

A buoyant LAM blade (100) is fixed to a shaft (101) and rotates on it.The blade projects beyond the waterline as indicated by the swell (11).The shaft (101) is held in bearings (not shown) that are fixed in afoundation (102). The foundation (102) is fixed to the seabed (14).Fixed to the shaft (101) and rotating with it is a power wheel (103).Wrapped round the power wheel is a power-transmitting cable (104). Theshaft (101) is aligned with the surge direction (89) and the blade (100)is tilted so that surge forces cause the blade (100) to move at rightangles to the surge. The power-transmitting cable (104) runs round atilted pulley (not shown) that turns the cable through a right angle.The cable runs to an OSG on the shore (see FIG. 9b ).

FIG. 10b : plan view showing blade cross-section.

The blade (100) is shown tilted with respect to the surge. The tilt isselected to give the best lift to drag ratio for the velocity ofoperation. Since the velocity of the flow past the blade (100) varieswith radial distance, the optimum blade (100) may be twisted.

FIGS. 10c and 10d : side views showing tuning with movable internalmasses.

The blade (100) is tuned by moving a dense internal mass (105). Tuningover a range of 3× is achieved by changing the radius of gyration by 3×.This is made possible by minimizing the fixed mass and the fixed addedmass of the blade (100) as already discussed. FIG. 10c shows the densemass (105) near the lower end of the blade (100). The period ofoscillation is 5 seconds. FIG. 10d shows the dense mass (105) near thetop of the blade (100). The period of oscillation is 15 seconds.

The incoming surge causes the blade (100) to rotate. The rotationcontinues until buoyancy forces overcome the surge force. As the surgeweakens, the rotation starts to reverse. As the surge reverses, theblade rotates in the opposite direction. The cycle repeats. The blademust be long enough to function through variations in tidal height. Inprinciple the blade can be any height that is required for the desiredrange of tuning.

FIG. 11a : side view in cross-section of omnidirectional surging WEC.

FIG. 11a shows a cross-section of a floating surging body (110) withfour OSGs (111) arranged with radial symmetry.

FIG. 11b : plan view in cross-section showing arrangement of multipleOSGs.

FIG. 11b shows a plan view in cross-section of a floating surging body(110) with three OSGs (111) arranged with radial symmetry.

The buoyant surging body (110) floats on the swell (11) in an offshorelocation. The body (110) is radially symmetrical and curved to capturesurge forces from any direction with minimum drag losses. A LAM disc isappropriate. In order to capture power from any direction, the body(110) has at least three OSGs (111) (no detail shown) fixedsymmetrically inside the body (110) at equal radial spacing and near thecircumference of the vessel (110). The OSGs (111) are driven by shafts(112) that pass through bearings (not shown) in the external wall of thesurging body (110) and bridge notches (113) in the circumference of thebody (110). Inside the notches (113) external wheels (114) are fixed to,and rotate upon, the shafts (112). Cables (115) are looped round thewheels (114). One end of each cable (115) is fixed to the seabed (14).The other end of each cable is fixed to a counterweight (116)

At the bottom center of the surging body (110) a position cable (117)passes through the floor of the body (110) and round a small, highlygeared electric winch (118) fixed to the interior of the body (110). Aflexible bellows (119) seals the cable to the body (110). Since therange of travel of the position cable is small, the bellows (119) needaccommodate only minor changes in length. The other end of the positioncable (117) is fixed to the seabed (14).

A swell from any direction tilts the surging body (110) and moves itsideways. As a result, the cables (115) connected to all three OSGs(111) move with respect to the body (110). As the surge reverses, thebody (110) reverts to its previous position. The cycle repeats.Reciprocating motion of the cables (115) is translated to electricalpower.

Tuning is by using the winch (118) to vary the water-plane area. Bydesign the floating body has high mass and high buoyancy so that thenatural water-plane area is small relative to the potential water-planearea. As a result the period of oscillation is 15 seconds. By pulling onthe position cable (117), the water-plane area is increased by 9× andthe period falls to 5 seconds

FIG. 12a : side view in cross-section of pitching LAM blade tuned bymovable masses with short period.

A pitching LM and LAM blade (120) floats on a swell (11). Internalbalanced dense movable masses (121) are close to the pitching axis. Thepitching period is short.

FIG. 12b : side view in cross-section of pitching LAM blade tuned bymovable masses with long period.

A pitching LM and LAM blade (120) floats on a swell (11). Internalbalanced dense movable masses (121) are close to the ends of the blade(120). The pitching period is long.

FIG. 12c : front view in cross-section of LAM blade, approximately toscale.

The blade (120) has an optimal fineness ratio.

FIG. 12d : plan view of pitching LAM blade tuned by variable angle fins:short period. The blade (120) has variable angle fins (122) at each end.The fins are aligned with the pitching motion and added mass is low.Therefore the pitching period is short.

FIG. 12d : plan view of pitching LAM blade tuned by variable angle fins:short period.

The blade (120) has variable angle fins (122) at each end. The fins areat right angles to the pitching motion and added mass is high. Thereforethe pitching period is long

FIG. 12f : side view of pitching LAM blade showing location of fins.

The fins (122) are mounted on the keel of the LAM blade (120).

FIG. 13a : plan view of pitching WEC comprising a cylinder with LAMblades attached.

A pitching floating body (130) comprises a cylinder (131) with attachedsymmetrical LAM blades fore (132) and aft (133). The cylinder (131)rotates on a shaft (134) that passes through bearings (135) fixed in theends of the cylinder (131) on the cylinder axis. The cylinder axis isalso the pitching axis. The shaft (134) on each side of the cylinder(131) connects to a rigid yoke (136) that passes under the front blade(132). Rigidly connected to the aft blade (133) is a vane (137) thatensures that the pitching body aligns with the dominant swell whentethered from the pitching axis. The vane (137) is a rigid thin verticalplate with low mass.

FIG. 13b : side view of the same showing tethering by crank arm to apseudo-stationary (PS) platform.

The pitching body (130) floats on the swell (11). The yoke (136) isattached to a buoyant crank arm comprising an upper crank arm (138) anda lower crank arm (139). The upper arm (138) is connected to the lowerarm (139) by a vertically acting hinge (1310). The lower arm (139) isconnected to a pseudo-stationary platform (1311) by a highly compliantjoint (1312) that is a combination of universal joint and rotatingbearing. The platform (1311) is tethered to the seabed (14) by mooringlines (1313). The two-arm construction of the crank arm enables it tofollow the orbital pitching motion of the cylinder (131). The highlycompliant joint (1312) enables the crank arm to accommodate rolling andyawing motion of the pitching body (130) and to rotate and align with adominant swell of varying direction.

FIG. 13c : plan view of PS platform.

The PS platform (1311) is tethered by at least three lines (1313) atequal radial spacing. Four lines (1313) are shown. The PS platform(1311) comprises a large mass with large drag surfaces (not shown) andnet buoyancy. The natural oscillation period of the PS platform greatlyexceeds the upper range of swell periods. To reduce wave disturbance itis preferably at least 5 meters below the surface.

As the body (130) pitches, there is relative rotating motion between theshaft (134) and the cylinder (131). This enables an OSG (not shown) tobe used inside the cylinder (131).

An alternative PTO system places the OSG in a water-tight housing on thePS platform (1311). A cable is fixed to the axis of the cylinder (131)and loops round a wheel on the OSG and terminates in a suspendedcounter-weight. This system is not shown but is indicated, for example,in FIG. 1. This method reduces the fixed mass of the floating body andso aids tuning.

The cylinder (131), or any other floating body with a constant radiusabout the pitching axis, has negligible added mass in pitching. This ishelpful in seeking wide variation in the vessel oscillation period. Byusing LM construction and LAM blades fore and aft and using the tuningmethods described in FIG. 12, oscillation periods in the range 5 to 15seconds are possible.

The scale of this body is limited to around 40 m length since at 60 mthere is bridging of 5 second swells. Scale is unlimited in the sensethat many 40 m devices can be used.

FIG. 14: side view of pitching cylinder WEC showing out-of-watersupport.

A floating cylinder (140) pitches on the swell (11). A shaft carried onbearings (141) in the axis of the cylinder (140) is connected by a yoke(142) to a submerged crank arm that is fixed to a PS platform (not shownbut see FIG. 13). The PTO system can be an OSG (not shown) either in thecylinder (140) or on the PS platform as already described. Fixed overthe cylinder is an LM structure (143) that carries movable dense masses(144) and at the ends are variable angle fins (145) on LAM blades (146).A vane can be attached to aid self-alignment to the dominant swell asshown in FIG. 13. 137). This is omitted.

In this configuration, fixed added mass is reduced by not using asubmerged blade. The LM structure can use any or all of the materialsand methods discussed for structures with high specific strengths.

Operation and tuning are as already described for FIG. 13. The scale islimited to around 40 m length.

FIG. 15a : side view of pitching cylinder WEC with gravity restoringforce

FIG. 15b : plan view of pitching cylinder WEC with gravity restoringforce

A floating cylinder (150) pitches on the swell (11). Fixed to theunderside of the cylinder (150) and aligned with the axis of thecylinder (150) is a rigid vertical keel spar (151) terminating in adense keel mass (152). The spar (151) and the keel mass (152) are bothstreamlined in the pitching plane. Fixed at an angle to the top of thecylinder are two symmetrical rigid tuning spars (153). Attached to thetuning spars (153) and sliding on them are two symmetrical dense movablemasses (154). The tuning spars (153) are supported by a LM structurethat exploits the strength of materials in tension. A vertical spar(155) is fixed to the top of the cylinder (150) and rigid elements undertension (156) connect the top of the vertical spar (155) to the ends ofthe two tuning spars (153). Other rigid elements under tension (157)connect the ends of the cylinder to the ends of the tuning spars. A vane(not shown) can be attached to aid alignment with the dominant swell.

A rigid yoke (159) turns on bearings (158) in the wall of the cylinder(150). As discussed under FIG. 13, the relative motion of the yoke (159)and the cylinder enables an OSG to be used either in the cylinder or ona PS platform (not shown).

Tuning is by sliding the dense masses (154) symmetrically about thepitching axis. To vary the period by 3× requires the radius of gyrationto vary by 3×. By design, the fixed mass and the fixed added mass arelow so that 3× variation is possible.

The controlling restoring force is not buoyancy but gravity. The ends ofthe device remain largely above the sea and so wave bridging does notoccur. There are no scale limits except those set by strengths ofmaterials.

In the event of extreme weather, first the WEC is claimed. Then tanks(not shown) in the cylinder (150) are flooded and the WEC is submergedto safety.

FIGS. 16a to 16d : side views of different configurations of awave-following WEC.

A rational design of a wave following WEC (a) does not proliferatesegments and (b) ensures that the segment length corresponding to thepeak of the annual wave energy spectrum is available. The peak periodfor energetic swells is typically 8 to 10 seconds. The optimum length istherefore around half a wavelength at these periods or 50 to 80 meters.In these figures, the selected ‘peak’ length is 70 m. The drawings areroughly to scale.

FIG. 16a : the wave-follower comprises three segments. Segment 1 (160)is 70 m long. Segment 2 (161) is 20 m long. Segment 3 (162) is also 20 mlong.

The fore part of segment 1 is tethered and has its own PTO. This is notshown.

The first link between segments 1 and 2 (163) is unlocked. The secondlink between segments 2 and 3 (164) is unlocked. This configuration istuned to 9 seconds and 5 seconds.

FIG. 16b . The second link (164) only is locked. The system is tuned to9 seconds and 7 seconds.

FIG. 16c . The first link only is locked. The system is tuned to 10seconds and 5 seconds.

FIG. 16d : Both links are locked. The system is tuned to 12 seconds

This design allows tuning from 5 to 12 seconds period. At 15 secondsperiod, the 12 second configuration will still be effective even if notoptimal.

FIG. 16e : plan view in cross-section of a segment locking mechanism.

A first floating segment (165) is linked to a second floating segment(166) by a rigid yoke (167). The yoke (167) rotates on a horizontallyacting hinge (168) on the first segment (165) and on a shaft (169) thatpasses through the second segment (166), carried on bearings (1610). Thelink is therefore a universal joint. Fixed to the shaft (169) inside thesecond segment (166) is a gear wheel (1611) that transmits power to anOSG gear train (not shown). Also fixed to the shaft (169) inside thesecond segment (166) is a disc (1612). On each side of the disc (1612)are brake pads (1613). The link between the two segments (165, 166) islocked by applying these pads (1613) to the disc (1612).

The invention claimed is:
 1. A wave energy converter dynamically tunableover a wide range, meaning over a significant part of the range ofperiods that characterise high energy swell, comprising: a. at least onebuoyant body arranged to oscillate under the influence of ocean swell;b. means for aligning said buoyant body with the dominant swell; c. apower take off system; d. a control system, comprising means forcollecting data on the prevailing sea-state and means for calculatingadjustments to the disposition and oscillation period of said buoyantbody; e. means for dynamically varying the oscillation period of saidbuoyant body said means being selected from at least one of: i varyingthe total mass of said buoyant body by adding or subtracting mass; iiarranging said buoyant body to oscillate by rotation about an axis andby arranging movement of mass to and from said axis along a radius tosaid axis; iii varying the water-plane area of said buoyant body; f.said variation in total mass of said buoyant body excluding significantpumping of seawater; g. said buoyant body being arranged tosignificantly limit the level of fixed mass moment of inertia (MOI) ofsaid buoyant body by means of streamlining in the direction ofoscillation of said buoyant body, whereby said tuning over a wide rangeis enabled; h. said buoyant body having a low fixed mass MOI meaning afixed mass MOI that is less than 20% of the maximum variable sum of massMOI and added mass MOI and preferably less than 10% of said maximum sum,whereby said tuning over a wide range is enabled; i. means forcontrolling bridging of waves by said buoyant body whereby limits totuning range can be controlled.
 2. A wave energy converter as claimed inclaim 1 wherein said total mass of said buoyant body can be varied bymeans of: a. at least one submerged mass that is substantially neutrallybuoyant; b. said submerged mass being substantially streamlined in thedirection of oscillation; c. a controllable variably compliant couplingbetween said buoyant body and said submerged mass; d. said compliantcoupling having a locked position wherein said coupling is a rigidconnection between said buoyant body and said submerged mass wherebysaid buoyant body and said submerged mass move together; e. saidcompliant coupling having an unlocked position wherein said coupling isdisconnected from said buoyant body, whereby said submerged mass floatsfreely.
 3. A wave energy converter as claimed in claim 2 wherein: a.said buoyant body is arranged to oscillate in the heaving vector; b.attached to the underside of said buoyant body is a rigid vertical rod;c. said submerged mass is an annular structure slidably attached to saidrod; d. said variably compliant coupling comprises at least one movablemember fixed inside said submerged mass and adjacent to said rod andcapable of being moved against said rod and attached to a poweredactuator; e. said submerged mass contains a controllable trimming tank,whereby the buoyancy of said submerged mass can be varied.
 4. A claim asin claim 1 wherein said varying of said total mass of said body is byvarying the added mass of said buoyant body, by using variable anglefins and the angle of said fins is variable continuously betweenalignment with said oscillating motion of said body and a position atright angles to said alignment.
 5. A claim as in claim 4 wherein saidvariable angle fins are located inside enclosures whereby the added masseffect and mass effect of said fins are increased.
 6. A claim as inclaim 1 wherein said varying of said total mass of said buoyant body isby trapping and releasing of seawater using means that avoid drag.
 7. Aclaim as in claim 6 wherein said means of avoiding drag is the closingof open-ended tubes at both ends with streamlined bodies, whereby thecombination of said tubes and said streamlined bodies is itself astreamlined body.
 8. A claim as in claim 1 wherein said varying of totalmass of said buoyant body is by locking and unlocking the links betweena series of floating bodies and said locking and unlocking provides atleast one rigid immersed length of floating body that substantiallyequals the half-wavelength of the swell period corresponding to peakannual swell energy at the location of said wave energy converter.
 9. Aclaim as in claim 8 wherein said locking and unlocking is by means ofbrakes to stop the rotation of a shaft, and said shaft passes throughthe interior of a first floating body and is fixed to a yoke thatconnects to a second floating body.
 10. A claim as in claim 1 whereinsaid buoyant body is arranged to oscillate by rotation about an axis byfixing said buoyant body to an arm that rotates on a substantially fixedaxis.
 11. A claim as in claim 1 wherein said low fixed mass moment ofinertia (MOI) derives from using at least one of: a. constructionmaterials with strength to weight ratios at least 2× conventionalstructural steel; b. sandwich composites with strength to weight ratiosat least 2× conventional structural steel; c. stressed skin methods ofconstruction; c. space frame methods of construction; d. locating heavycomponents of said wave energy converter outside said buoyant body. 12.A claim as in claim 1 wherein said level of fixed added mass MOI of saidbuoyant body is significantly limited, meaning a fixed added mass MOIthat is less than 20% of the fixed mass MOI of said buoyant body andpreferably less than 10%, whereby said tuning over a wide range isenabled.
 13. A claim as in claim 12 wherein said significantly limitedadded mass MOI is achieved by using at least one of: a. streamlining ofsaid buoyant body in the direction of oscillation and said streamliningemploys a fineness ratio in the range of 3 to 10 and preferably 4.5; b.said buoyant body is arranged to pitch and has a submerged exteriorsurface defined by one or more constant radii about the pitching axis;c. said buoyant body is arranged to pitch and a substantial part of thepitching extremities of said buoyant body is arranged to be out of thewater.
 14. A claim as in claim 1 wherein said varying of saidwater-plane area of said buoyant body is by changing the verticaltension of a link between said buoyant body and a fixed submergedsurface.
 15. A claim as in claim 1 wherein said bridging of waves iscontrolled by at least one of: a. limiting the immersed length of saidbuoyant body to less than a single wave-length at the shortest desiredperiod of oscillation of said buoyant body; b. varying the immersedlength of said buoyant body by locking and unlocking adjacent serialsegments; c. applying a gravitational restoring force to said buoyantbody, whereby immersion of buoyant extremities of said buoyant body isavoided.
 16. A claim as in claim 15 wherein said gravitational restoringforce is provided by a weighted keel, streamlined in the pitching plane,fixed below said buoyant body that is a pitching cylinder pitching aboutthe cylindrical axis and attached to said cylinder is an upward-angledsymmetrical structure arranged to be out of the water and arranged sothat masses kept in balance about the pitching axis can be moved to andfrom said axis, whereby dynamic tuning over a wide range is enabled. 17.A claim as in claim 1 wherein said means for aligning said buoyant bodywith said dominant swell is one of: a. a near-shore location of saidbody where said dominant swell has persistent direction; b. said bodyhaving radial symmetry whereby said body interacts equally with swellfrom all directions; c. means for automatic alignment with said dominantswell.
 18. A claim as in claim 17 wherein said means for automaticalignment is one of: a. said buoyant body is arranged to pitch and istethered by a yoke and said yoke is fixed to the fore section of saidbuoyant body; b. said buoyant body is arranged to pitch and is tetheredby a yoke at its pitching axis and said yoke is arranged to rotate aboutsaid axis.
 19. A claim as in claim 18 wherein said yoke arranged torotate is attached to a crank arm comprising two sections wherein thetwo sections are joined by a vertically acting hinge and the lowestsection is fixed to a submerged pseudo-stationary platform by acombination of universal joint and horizontally rotating bearing.
 20. Aclaim as in claim 1 wherein said power take off system comprises arotating shaft that is coupled to a rectifying ratchet wheel that iscoupled to a speed-multiplying gear train that is coupled to anoverrunning clutch that is coupled to a flywheel that is coupled to arotary electrical generator, said rotating shaft being driven by one of:i. the relative motion of said buoyant body and a cable attached to asuspended counterweight; ii. the relative motion of said buoyant bodyand a cable attached to a another body; iii the relative motion of saidbuoyant body and an arm attached to another body.